Hydrogen absorbing alloy electrodes must be great in discharge capacity and great in the degree of initial activation of the hydrogen absorbing alloy and have a prolonged charge-discharge cycle life. Accordingly, it has been proposed to prepare electrodes from a mixture of different kinds of hydrogen absorbing alloys so as to give the electrode the characteristics of the different alloys (for example, JP-A-129936/1989, -173062/1991 and -162355/1992).
For example, hydrogen absorbing alloys with an AB.sub.5 -type crystal structure have a relatively great degree of initial activation and a relatively long charge-discharge cycle life, but are small in discharge capacity. On the other hand, hydrogen absorbing alloys having an AB.sub.2 -type crystal structure are relatively great in discharge capacity but small in the degree of initial activation and short in charge-discharge cycle life. It is therefore practice to prepare hydrogen absorbing alloy electrodes by mixing together a hydrogen absorbing alloy of AB.sub.5 -type crystal structure and a hydrogen absorbing alloy of AB.sub.2 -type crystal structure, molding the mixture and sintering the molded mixture.
Furthermore, AB.sub.2 -type hydrogen absorbing alloys having a Zr--Ni Laves-phase structure are known as alloys having a great discharge capacity. While such alloys vary in properties depending on the composition, the degree of initial activation and the charge-discharge cycle life thereof are generally in trade-off relationship with each other; the alloys excellent in one of these properties are inferior in the other. Accordingly, an AB.sub.2 -type alloy (e.g., Zr--Mn--V--Ni alloy) of a composition ensuring a great initial activation degree is mixed with another AB.sub.2 -type alloy (e.g., Zr--Ti--V--Ni alloy) of a composition having a long charge-discharge cycle life to prepare hydrogen absorbing alloy electrodes from the mixture. The electrodes obtained are improved in both the properties of initial activation degree and charge-discharge cycle life.
In the case where electrodes are prepared from a sintered mixture of AB.sub.5 -type hydrogen absorbing alloy and AB.sub.2 -type hydrogen absorbing alloy as conventionally practiced (JP-A-162355/1992), or in the case of hydrogen absorbing alloy electrodes prepared from an alloy wherein the surfaces of AB.sub.2 -type hydrogen absorbing alloy particles are covered with finer AB.sub.5 -type hydrogen absorbing alloy particles (JP-A-173062/1991), joint interfaces are formed between the AB.sub.2 -type hydrogen absorbing alloy particles and the AB.sub.5 -type hydrogen absorbing alloy particles, whereas the joint interfaces are not exposed on the surfaces where hydrogen absorption and desorption reactions proceed, so that the electrodes are not effectively improved in initial activation and charge-discharge cycle life. Alternatively when one of AB.sub.5 -type hydrogen absorbing alloy and AB.sub.2 -type hydrogen absorbing alloy is coated with an electrically conductive metal or ceramic and then mixed with the other alloy in a bare state, followed by sintering to prepare electrodes from the sintered mixture, the electrodes fail to exhibit a fully improved initial activation degree since joint interfaces are not satisfactorily formed between AB.sub.5 -type hydrogen absorbing alloy particles and AB.sub.2 -type hydrogen absorbing alloy particles owing to the coating.
When hydrogen absorbing alloy electrodes are prepared by mixing together hydrogen absorbing alloys of different crystal structures and sintering the mixture as in the prior art, the characteristics available are merely approximate to the average of those of the alloys. Thus, the electrodes fail to exhibit outstanding characteristics synergistically resulting from the characteristics of the component hydrogen absorbing alloys.
Further hydrogen absorbing alloy electrodes prepared from a mixture of hydrogen absorbing alloys which are merely different in kind as heretofore practiced have characteristics which are also approximate to the average of the characteristics of the different alloys, and it is impossible to obtain characteristics synergistically resulting from the different hydrogen absorbing alloys.